As shown in FIGS. 10 to 12, reference symbol P denotes a reading original P placed on an original glass table 100. The image of the original is read by scanning a reading unit 101 parallel to the original glass table 100. As shown in FIG. 6, three color LEDs 101R, 101G, and 101B serving as the light sources for irradiating the original, a rod lens array 101L for causing light reflected by the original to form an image on the light-receiving element of an image sensor 101S, and the image sensor 101S are built into the reading unit 101. Color separation reading is performed by sequentially turning on the three color light sources by switching operation and reading light of respective colors reflected by the original with the image sensor 101S.
Reference numeral 111 denotes a frame serving also as an outer cover. In addition to the original glass table 100 and reading unit 101, a rail 112 for guiding travel of the reading unit 101, a control board, a power supply, and the like are arranged in the frame 111.
Spacers 102 made of a highly slidable material, e.g., POM, are fixed to the two ends in the main scanning direction of the upper surface of the reading unit 101. Support shafts 101a and 101b respectively arranged near the ends in the main scanning direction of the reading unit 101 are supported by U-shaped portions 103a and 103b on a sensor holder 103 to be vertically movable, and are biased upward by springs 107a and 107b fixed on the sensor holder 103. Consequently, the spacers 102 are in contact with the lower surface of the original glass table 100, so the reading unit 101 scans the original while maintaining a constant distance between the glass surface and the light-receiving surface of the image sensor 101S.
The sensor holder 103 has slider portions 103c and 103d which slide with respect to the rail 112. The slider portions 103c and 103d are made of a material such as POM, in the same manner as the spacers 102.
As shown in detail in FIGS. 13 to 15, a pulse motor 104 serving as a transmission mechanism for transmitting a driving force to the reading unit 101 and as a driving source, a gear train for decelerating rotation of the pulse motor 104, a driving pulley 105 with a tooth portion 105a corresponding to the spiral of a driving wire 113 (to be described later) and to which rotation from the motor is transmitted through the gear train, and an idle pulley 106 are rotatably arranged on the sensor holder 103.
The reading unit 101 is connected to the control board on the main body through a cable (not shown) and receives power and a driving signal from it and sends an image signal to it.
The rail 112 for guiding travel of the reading unit 101 is stationarily placed on the frame 111. One end 113a of the driving wire 113 is fixed to the reading terminal end of the apparatus frame 111. The driving wire 113 is wound around the driving pulley 105 on the reading unit 101, is looped on the idle pulley 106, and is fixed to the reading start end of the apparatus frame 111 through a spring 114.
When the driving wire 113 is set in the above manner, the moment indicated by arrows A acts on the reading unit 101, so the slider portions 103c and 103d on the sensor holder 103 abut against the rail 112.
In the inoperative state, the reading unit 101 usually waits at the home position on the reading start end. Upon reception of a reading instruction from a computer connected to it, the reading unit 101 starts reading by rotation of the motor, scans a white reference plate arranged between the home position of the apparatus and the original reading start position to generate shading correction data, and reads the image on the original starting from the reading start position.
Rotation of the motor is decelerated through the gear train, and is transmitted to the driving pulley 105. Usually, the stepping angle of the motor, the reduction ratio of the gear, and the outer diameter of the driving pulley are determined such that the reading unit 101 moves for a distance corresponding to one subscanning line in response to a plurality of driving pulses supplied to the motor. When the motor rotates in the forward direction, the driving pulley 105 takes up the wire, and accordingly the reading unit 101 moves in the scanning direction. When the motor rotates in the reverse direction, the reading unit 101 moves toward the home position.
The above prior art has the following problems.
As described above, the reading unit 101 is biased toward the original glass table 100 by the springs 107a and 107b arranged at the two ends of the sensor holder 103 which is supported substantially at its center by the guide rail 112. These springs 107a and 107b must be able to uniformly urge the right and left ends of the reading unit 101 against the original glass table 100.
From the viewpoint of assembly, the springs 107a and 107b are preferably identical, so an error in attaching can be prevented.
Due to the arrangement of the apparatus, however, it is difficult to maintain an urging pressure against the original glass table 100 uniform on the right and left sides by using the identical springs.
For example, in the above case, since that side of the sensor holder 103 which has the driving transmitting system is heavier than the other side thereof with respect to the guide rail 112, the sensor holder 103 is undesirably tilted. To uniformly urge the reading unit 101 under this condition, the load of the spring on the driving transmission system side must be increased. Also, tilt itself of the sensor holder 103 poses a problem.